How to shut down a wind turbine

Abstract

The present invention provides a method for shutting down a wind turbine, the wind turbine comprising: a rotor having a plurality of blades; and a generator system coupled to the rotor. The method comprises: operating the generator system to generate electric power 5 and applying a load torque to the rotor; using a power reference signal, controlling the generated electric power; determining that the wind turbine needs to be shut down; in response to determining that the wind turbine needs to be shut down, changing the power reference signal to increase the generated electric power, thereby slowing down the rotor; determining that the speed of the rotor has decreased 10 below a threshold; and in response to determining that the speed of the rotor has decreased below the threshold, changing the pitch of the blades to further slow down the rotor.

Description

Technical Field

[0001] The present invention generally relates to a method of shutting down a wind turbine. Background Art

[0002] Traditionally, in emergency events, such as high yaw error, the wind turbine is shut down by pitching the wind turbine blades upwind to immediately slow the rotor.

[0003] This approach can result in large asymmetric loads on the blades.

[0004] Accordingly, there is a need for methods of shutting down wind turbines that avoid or mitigate such loads. Summary of the invention

[0005] A first aspect of the present invention provides a method for shutting down a wind turbine, the wind turbine comprising: a rotor having a plurality of blades; and a generator system coupled to the rotor, the method comprising: operating the generator system to generate electrical power and applying a load torque to the rotor; controlling the generated electrical power using a power reference signal; determining that the wind turbine needs to be shut down; in response to determining that the wind turbine needs to be shut down, changing the power reference signal to increase the generated electrical power, thereby slowing down the rotor; determining that the rotor speed has dropped below a threshold; and in response to determining that the rotor speed has dropped below the threshold, changing the pitch of the blades to further slow down the rotor.

[0006] Controlling the generated electric power indirectly controls the load torque applied to the wind turbine because there is a relationship between the generated electric power and the load torque applied by the generator.The threshold value may be a predetermined threshold value.

[0007] The generator system may include: a generator and an electric converter; electric power and load torque generated by the generator; the electric converter converts the frequency of the electric power to generate a grid signal input to the grid; and a power reference signal that controls the electric converter, which in turn controls the electric power and load torque generated by the generator.

[0008] The pitch of the blades may be maintained substantially constant until the speed of the rotor has decreased below a threshold value.

[0009] As the power reference signal changes in response to determining that the wind turbine needs to be shut down, the pitch of the blades may be maintained substantially constant.

[0010] From the time it is determined that the wind turbine needs to be shut down, the pitch of the blades may be kept substantially constant until the rotor speed has dropped below a threshold value.

[0011] The term "substantially constant" as used above may mean that the pitch does not change by more than a small amount, for example 2°, 5° or 10°.

[0012] The power reference signal may be changed by an amount determined, at least in part, based on the load torque limit so that the load torque applied to the rotor does not exceed the load torque limit.

[0013] After determining that the rotor speed has dropped below a threshold, the power reference signal may be changed to reduce the generated electrical power. For example, the power reference signal may be reduced to reduce the electrical power to zero.

[0014] Determining that shutdown is required may include identifying an error condition. The error condition may be a yaw error condition. Based on the error condition, a threshold may be determined. In other words, based on the cause of the shutdown, a threshold may be set.

[0015] The threshold may be a percentage of an initial speed, for example the rotor speed when the power reference signal is changed. Alternatively, the threshold may be an absolute speed.

[0016] Changing the pitch of the blades may slow the rotor to a stop. Alternatively, changing the pitch of the blades may slow the rotor to idle speed.

[0017] Changing the blade pitch to further slow the rotor may change the pitch by more than 30°, more than 50°, or more than 70°.

[0018] Another aspect of the present invention provides a wind turbine, comprising: a rotor having a plurality of blades; a generator system coupled to the rotor; and a control system, wherein the control system is configured to: operate the generator system to generate electric power and apply a load torque to the rotor; control the generated electric power and load torque using a power reference signal; determine that the wind turbine needs to be shut down; in response to determining that the wind turbine needs to be shut down, change the power reference signal to increase the generated electric power and load torque, the increased load torque slowing down the rotor; determine that the rotor speed has dropped below a threshold; and in response to determining that the rotor speed has dropped below the threshold, change the pitch of the blades to further slow down the rotor.

[0019] The control system may be further configured to: determine that a maximum time has been exceeded after the power reference signal has changed (e.g., a maximum time measured relative to the time when the power reference signal has changed, or relative to the time when it was determined that the wind turbine needed to be shut down); and in response to determining that the maximum time has been exceeded, change the pitch of the blades to slow the rotor.

[0020] The control system may be further configured to: monitor the speed of the rotor to identify abnormal behavior (eg, speed increasing rather than decreasing) after the power reference signal has been changed; and in response to identifying the abnormal behavior, change the pitch of the blades to slow the rotor.

[0021] Another aspect of the present invention provides a computer-readable storage medium having computer-readable program code embodied therein, the computer-readable program code being executable by one or more computer processors to perform operations on a wind turbine, the wind turbine comprising: a rotor having a plurality of blades; and a generator system coupled to the rotor, wherein the operations comprise: operating the generator system to generate electric power and to apply a load torque to the rotor; controlling the generated electric power and the load torque using a power reference signal; determining that the wind turbine needs to be shut down; in response to determining that the wind turbine needs to be shut down, changing the power reference signal to increase the generated electric power and the load torque, the increased load torque slowing the rotor; determining that the rotor speed has dropped below a threshold; and in response to determining that the rotor speed has dropped below the threshold, changing the pitch of the blades to further slow the rotor.

[0022] Another aspect of the present invention provides a method for shutting down a wind turbine, the wind turbine comprising: a rotor having a plurality of blades; and a generator system coupled to the rotor, the method comprising: operating the generator system to generate electrical power and to apply a load torque to the rotor; utilizing a power reference signal, controlling the generated electrical power; determining that the wind turbine needs to be shut down; in response to determining that the wind turbine needs to be shut down, changing the power reference signal to increase the generated electrical power; and after the power reference signal has been changed, determining that a maximum time has been exceeded; and in response to determining that the maximum time has been exceeded, changing the pitch of the blades to slow the rotor.

[0023] Another aspect of the present invention provides a method for shutting down a wind turbine, the wind turbine comprising: a rotor having a plurality of blades; and a generator system coupled to the rotor, the method comprising: operating the generator system to generate electrical power and applying a load torque to the rotor; utilizing a power reference signal to control the generated electrical power; determining that the wind turbine needs to be shut down; in response to determining that the wind turbine needs to be shut down, changing the power reference signal to increase the generated electrical power; monitoring the speed of the rotor after the power reference signal has been changed to identify abnormal behavior; and in response to identifying the abnormal behavior, changing the pitch of the blades to slow the rotor. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Embodiments of the present invention will now be described with reference to the accompanying drawings, in which:

[0025] Figure 1 is a schematic diagram of a wind turbine;

[0026] Figure 2 is a schematic diagram of a control system for a wind turbine; and

[0027] Figure 3 Various turbine parameters are shown during shutdown. DETAILED DESCRIPTION

[0028] Figure 1 An example of a wind turbine 1 is shown in a schematic perspective view. The wind turbine 1 includes a tower 2, a nacelle 3 at the top of the tower, and a rotor 4 operatively coupled to a generator contained inside the nacelle 3. In addition to the generator, the nacelle contains various components required for converting wind energy into electrical energy, as well as various components required for operating, controlling and optimizing the performance of the wind turbine 1. The rotor 4 includes a central hub 5 and a plurality of blades 6 extending outwardly from the central hub 5. In the embodiment shown, the rotor 4 includes three blades 6, but the number can vary, for example, the rotor can have two or four blades. In addition, the wind turbine includes a control system. The control system can be placed inside the nacelle or distributed at multiple locations inside the turbine and communicatively connected.

[0029] The wind turbine 1 may be included in a collection of other wind turbines belonging to a wind power plant (also called a wind farm or wind farm) which acts as a power plant connected to a power grid via transmission lines. The power grid generally consists of a network of power stations, transmission lines and substations coupled by a network of transmission lines which transmit power to loads in the form of end users and other customers of the electric utility.

[0030] Figure 2 An embodiment of a control system 20 together with wind turbine elements is schematically shown. The rotor blades 6 are mechanically connected to a generator 22 via a gearbox 23 with an input shaft and an output shaft 21. In direct drive systems and other systems, the gearbox 23 may not be present. The electrical power generated by the generator 22 is injected into a power grid 24 via an electrical converter 25. The generator 22 and the converter 25 may be based on a full scale converter (FSC) architecture or a double fed induction generator (DFIG) architecture, but other types may also be used.

[0031] The control system 20 includes a plurality of elements, including at least one main controller 200 having a processor and a memory so that the processor can perform computational tasks based on instructions stored in the memory. Generally, the wind turbine controller ensures that the wind turbine generates the required power output level during operation. This is achieved by adjusting the pitch angle and / or the power extraction of the converter 25. To this end, the control system includes: a pitch system, which includes a pitch controller 27 using a pitch reference signal 28, and a power system, which includes a power controller 29 using a power reference signal 26. The power controller 29 controls the various electrical components of the generator converter system to deliver the required power, which in turn controls the torque of the generator 22, which is required for the rotor to extract the required power from the wind.

[0032] The torque of the generator 22 (hereinafter referred to as generator torque) is applied to the output shaft 21 of the transmission 23 .

[0033] The rotor blades 6 can be pitched by a pitch control mechanism. The rotor may include a common pitch system that adjusts all pitch angles on all rotor blades simultaneously, and in addition individual pitch systems that are capable of pitching the rotor blades individually. The control system 20, or elements of the control system 20, may be placed in a power plant controller (not shown) so that the turbine can be operated based on externally provided instructions. In an embodiment of the invention, the pitch is controlled based on a common pitch reference (on which the individual pitch references are superimposed). In an embodiment, slowing down the rotor can be achieved by changing the common pitch reference. And keeping the pitch of the blades constant can achieve keeping the common pitch reference constant.

[0034] To ensure that the power reference signal 26 does not damage the power controller 29 or the converter 25, the power reference signal 26 is limited in magnitude such that it cannot exceed a predetermined maximum value, such as a maximum voltage value.

[0035] The rotor 4 is subject to an aerodynamic torque and a load torque acting in opposite directions. The aerodynamic torque is a result of the wind acting on the rotor blades 6. Changing the pitch of the blades changes the aerodynamic torque, which is achieved by changing the angle of attack and thereby changing the aerodynamic lift. The load torque acts on the rotor against the aerodynamic torque. The load torque has a partial amount from the generator torque applied to the output shaft 21 of the gearbox, and a component caused by friction in the gearbox 23 and the bearings.

[0036] The difference between the aerodynamic torque and the load torque is the net torque applied to the rotor. If the net torque is zero, i.e. the aerodynamic torque and the load torque are equal, then the speed of the rotor will not change. If the aerodynamic torque is greater than the load torque, the rotor will speed up. If the load torque is greater than the aerodynamic torque, the rotor will slow down.

[0037] As explained above, the generator torque produces most of the load torque applied to the rotor. Therefore, requesting more power from the generator 22 results in a greater load torque being borne by the rotor 4. The relationship between torque and power is: P = ω × T, where P is the power generated, T is the load torque, and ω is the speed of the rotor.

[0038] During normal operation of the wind turbine, the speed of the rotor is controlled using the pitch angle of the blades and / or the load torque, which are controlled by the pitch reference signal 28 and the power reference signal 26, respectively.

[0039] In the low wind speed operating region, the pitch reference signal 28 may be held constant, and the power reference signal 26 and the rotor speed may be increased as the wind speed increases. In the high wind operating region, the power reference signal 26 and the rotor speed may be held constant (at rated power and rated speed, respectively), and the pitch reference signal 28 may be adjusted in response to changes in wind speed.

[0040] In an emergency, conventionally, a wind turbine can be shut down by pitching out the blades. However, pitching out the blades results in a large change in the load distribution experienced by the blade surface. This is because different parts along the length of the blade experience different lift coefficient distributions as the blade completes its rotation. When the angle of attack changes during the pitching action, the lift coefficient distribution changes differently for each part of the blade, which means that the entire surface of the blade experiences inconsistent lift coefficient changes. As a result, the load distribution on the blade surface can experience large changes. This is undesirable as it can cause damage or fatigue to parts of the wind turbine, such as bearings.

[0041] The present invention avoids or mitigates these problems by: determining that the wind turbine needs to be shut down; then in response to determining that the wind turbine needs to be shut down, changing the power reference signal 26 to increase the generated electrical power, thereby slowing the rotor; determining that the rotor speed has decreased below a speed threshold; and in response to determining that the rotor speed has decreased below the threshold, changing the pitch of the blades (by increasing the pitch reference signal 28) to further slow the rotor.

[0042] An increase in the power reference signal reduces the rotor speed using the principle described above, i.e. increasing the power generated, which in turn increases the load torque applied to the rotor. The aerodynamic torque does not change because the wind speed does not change much, and the blade pitch does not change. Therefore, the net torque causes the rotor to slow down.

[0043] A wind direction sensor 30, such as a wind vane, may be used to determine that a wind turbine shutdown is required due to high yaw error. The wind direction sensor 30 determines the angle of the wind relative to the nacelle. If the magnitude of the angle increases above a threshold angle, a high yaw error is determined to exist, thereby triggering a shutdown.

[0044] Other possible reasons for determining the need to shut down a wind turbine include: extreme wind events; sensor, actuator or component failure; or loss of connection to the grid.

[0045] Figure 3 This technique is illustrated, which shows examples of system parameters when employing the present invention.

[0046] Normal operation of the wind turbine occurs until time t0. During normal operation before time t0, the generated electrical power is controlled using the power reference signal 26 and / or the pitch reference signal 28 as described above.

[0047] At time t0, conditions change. For example, the condition may be that the wind direction changes so that the wind is no longer aligned with the rotor, and instead the wind enters the turbine from the side. Thus, at time t0, the yaw error signal begins to increase, as shown at 40.

[0048] As the yaw error increases, the power reference signal 26, power, the pitch reference signal 28 and the load torque all decrease, as shown at 1, 42, 43 and 44 respectively. The generator speed decreases and then recovers, as shown at 45.

[0049] At time t1, it is determined by the controller 200 that the wind turbine needs to be shut down because the yaw error signal has exceeded a threshold value 50. In response to determining that the wind turbine needs to be shut down, the power reference signal 26 is increased (as shown at 51) to increase the generated electrical power (as shown at 52). The wind speed does not change much, and the pitch of the blades does not change (as shown at 54, the constant pitch reference signal), so the aerodynamic torque does not change. The increased power demand causes the load torque to increase (as shown at 55). Since the aerodynamic torque has not changed, the net torque now applied causes the rotor to slow down (as shown at 53).

[0050] The power reference signal 26 increases until reaching the power offset level 60 at time t2. The power offset level 60 may be determined in a variety of ways.

[0051] In one example, the power reference signal 26 may vary by a fixed and predetermined amount (eg, 1 MW).

[0052] In another example, the power reference signal 26 may vary by a fixed percentage (eg, 50%, 100%, or 200%).

[0053] In other examples, power reference signal 26 may vary by an amount that is dependent upon various parameters (eg, load torque, blade pitch, and power).

[0054] Alternatively, the power reference signal 26 may be changed by an amount determined, at least in part, based on the load torque limit so that as the speed becomes slower, the load torque applied to the rotor does not exceed the load torque limit (torque=power / speed). Thus, for example, if increasing the power reference signal 26 by 1 MW would cause the load torque to exceed the load torque limit, the power reference signal 26 is increased by a smaller amount that maintains the load torque below the load torque limit.

[0055] Alternatively, the power reference signal 26 may be varied by a proportional-integral (PI) controller that attempts to reach a reference rate (eg, rate threshold 70 mentioned below) using power with load torque limitation and associated integrator windup protection.

[0056] Alternatively, the controller 200 may look at generator speed, tower oscillations, blade loading, etc., and control the power reference signal 26 accordingly.

[0057] Between time t2 and time t3, the power reference signal 26 remains constant (as shown at 61), and therefore the power also remains constant (as shown at 62). Since the pitch reference signal 28 also remains constant during this phase (as shown at 63), the load torque begins to decrease slightly (as shown at 64).

[0058] At time t3, controller 200 determines that the rotor speed has dropped below speed threshold 70. This determination may be made by monitoring the speed of the rotor or generator using one or more sensors.

[0059] Determining that the speed of the rotor 4 has decreased below the speed threshold 70 triggers an increase in the pitch reference signal (as shown by ramp 71), causing the pitch of the blades to increase until it reaches 90°. This reduces the aerodynamic torque, which serves to further slow the rotor (as shown at 72). In this case, the rotor slows to a complete stop. In other cases, the rotor may slow to idle speed.

[0060] The speed threshold 70 may be a percentage of the initial speed of the rotor at time t1, or it may be an absolute value. The threshold speed 70 may be predetermined, or it may be determined by the controller 200 during the aforementioned shutdown process.

[0061] Note that the pitch reference signal 28 is shown as being constant between time t1 and t3. Thus, the pitch of the blades remains substantially constant when the power reference signal 26 changes between time t1 and time t2, and also remains substantially constant from the time it is determined that the wind turbine needs to be shut down (time t1) until the speed of the rotor has dropped below a threshold value (time t3).

[0062] Note, however, that during the time between time t1 and time t3, the individual pitch signals of each blade may vary slightly (e.g., a few degrees), while the collective pitch angle may vary by a smaller amount. On the other hand, the change in pitch angle caused by ramp 71 is larger (e.g., it may be greater than 40° or greater than 60°). Thus, the term "substantially constant" used above may mean that the pitch does not vary by more than a small amount, such as 2°, 5°, or 10°.

[0063] At time t3, the power reference signal 26 and power decrease, as shown at 72 and 73 respectively, and the load torque decreases, as shown at 74. As the generator speed decreases, the generator cannot generate as much power due to the relationship P = ω x T.

[0064] As the power reference signal 26 begins to increase at time t1, the speed should decrease rapidly as shown at 53. If the speed does not decrease as expected, it may be desirable to change the pitch of the blades to shut down the rotor even if the speed does not decrease below the speed threshold 70. This can be addressed in a number of ways.

[0065] In a first example, the controller 200 may be configured to determine that a maximum time has been exceeded; and in response to determining that the maximum time has been exceeded, change the pitch of the blades to slow down the rotor. For example, the maximum time may be measured relative to the time t1 at which the power reference signal has increased, in which case it is also the time t1 at which it is determined that the wind turbine needs to be shut down.

[0066] In a second example, the controller 200 may be configured to monitor the speed of the rotor after the power reference signal 26 has increased at time t1 to identify abnormal behavior. For example, the speed may increase rather than decrease. In response to identifying this abnormal behavior, the pitch of the blades may be changed immediately to slow the rotor, rather than waiting for the speed to drop below the speed threshold 70.

[0067] An advantage of the above procedure is that the mechanical forces acting on the wind turbine are limited. After the power reference signal has increased at t1, the absolute values ​​of both the pitch moment and the yaw moment on the main bearing are significantly reduced, thereby reducing the mechanical loads on the wind turbine.

[0068] Other advantages include reduced negative thrust peaks, which results in lower tower loading and lower negative blade wing loading, and reduced required maximum pitch rate (ie the rate of change of pitch angle caused by ramp 71 ), which results in lower loads on the pitch control system.

[0069] Although the invention has been described above with reference to one or more preferred embodiments, it will be appreciated that various changes or modifications may be made without departing from the scope of the invention as defined in the appended claims.

Claims

1. A method of shutting down a wind turbine, the wind turbine comprising: a rotor having a plurality of blades; and a generator system coupled to the rotor; The method comprises: operating a generator system to generate electrical power and to apply a load torque to a rotor; controlling the generated electrical power using a power reference signal; Determining the need to shut down wind turbines; in response to determining that the wind turbine needs to be shut down, changing the power reference signal to increase the generated electrical power, thereby slowing the rotor, wherein the pitch of the plurality of blades remains substantially constant as the power reference signal changes; determining that the rotor speed has decreased below a threshold; and In response to determining that the rotor speed has decreased below a threshold, the pitch of the plurality of blades is changed to further slow the rotor.

2. The method according to claim 1, wherein: From the time it is determined that the wind turbine needs to be shut down, the pitch of the plurality of blades is maintained substantially constant until the rotor speed has dropped below a threshold value.

3. A method according to any one of the preceding claims, wherein: Determining that a shutdown is needed includes identifying an error condition.

4. The method according to claim 3, wherein: The error condition is a yaw error condition.

5. The method according to claim 3, wherein: Based on the error condition, a threshold value is determined.

6. The method according to any one of claims 1 to 2, wherein: The threshold is a percentage of the initial speed of the rotor.

7. The method according to any one of claims 1 to 2, wherein: The threshold is an absolute rate.

8. The method according to any one of claims 1 to 2, wherein: Changing the pitch of the blades slows the rotor to a stop.

9. The method according to any one of claims 1 to 2, wherein: An amount by which the power reference signal is changed is determined, at least in part, based on a load torque limit such that a load torque applied to the rotor does not exceed the load torque limit.

10. The method according to any one of claims 1-2, further comprising: After determining that the speed of the rotor has dropped below a threshold, the power reference signal is changed to reduce the generated electrical power.

11. Wind turbines, including: a rotor having a plurality of blades; a generator system coupled to the rotor; and A control system, wherein the control system is configured to: operating a generator system to generate electrical power and to apply a load torque to a rotor; Using the power reference signal, controlling the generated electric power and load torque; Determining the need to shut down wind turbines; In response to determining that the wind turbine needs to be shut down, changing a power reference signal to increase the generated electrical power and load torque, the increased load torque slowing the rotor, wherein the pitch of the plurality of blades remains substantially constant as the power reference signal changes; determining that the speed of the rotor has decreased below a threshold; and In response to determining that the speed of the rotor has decreased below a threshold, the pitch of the plurality of blades is changed to further slow the rotor.

12. The wind turbine according to claim 11, wherein: The control system is further configured to: determining that a maximum time has been exceeded after the power reference signal has changed; and In response to determining that the maximum time has been exceeded, the pitch of the blades is changed to slow the rotor.

13. A wind turbine according to claim 11 or 12, wherein: The control system is further configured as: After the power reference signal has been changed, monitoring the rotor speed to identify abnormal behavior; and In response to identifying the abnormal behavior, the pitch of the blades is changed to slow the rotor.

14. A computer readable storage medium having embodied therein computer readable program code executable by one or more computer processors to perform operations on a wind turbine, the wind turbine comprising: a rotor having a plurality of blades; and a generator system coupled to the rotor; wherein the operations include: operating a generator system to generate electrical power and to apply a load torque to a rotor; Using the power reference signal, controlling the generated electric power and load torque; Determining the need to shut down wind turbines; In response to determining that the wind turbine needs to be shut down, changing the power reference signal to increase the generated electrical power and the load torque, the increased load torque slowing the rotor, wherein the pitch of the plurality of blades remains substantially constant as the power reference signal changes; determining that the rotor speed has decreased below a threshold; and In response to determining that the rotor speed has decreased below a threshold, the pitch of the plurality of blades is changed to further slow the rotor.

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